Location of HETG/LETG at XRCF
                -----------------------------
                 Dan Dewey, 10/24/96, 11/2/96, 12/12/96

Summary
- - - -
The as-installed Rowland Diameter at XRCF is: 8782.8 mm.

This is 24.4 mm smaller than ideal, some (4%) additional vignetting
will occur (primarilly LETG shell 1).

For the csim/MARX simulator the parameter RowlandDiameter must be
set to 8590.0 mm to correctly model the system and achieve the effective
Rowland Diameter above.


Flight Grating location
- - - - - - - - - - - -
In flight the HRMA has a focal length of F_flight and the Rowland
Diameter is X_flight.  Current values for these (*1,2) are 10065.5 mm
and 8633.7 mm.


XRCF Grating location
- - - - - - - - - - -
At XRCF the HRMA will have its finite source focus at F_xrcf and the
gratings will be mounted with a Rowland Diameter of X_xrcf.  The ideal
value of X_xrcf - in order that the HRMA rays go through the center of
the gratings - is obtained from similar triangles using the line from
the center of the hyperboloid to the focus (which just about goes
through the grating centers) :

                     ( F_xrcf - Node_Hcenter )
  X_xrcf = X_flight ---------------------------
                     ( F_flight - Node_Hcenter )

where Node_Hcenter is the axial distance from the HRMA node to the
center of the Hyperboloids (in the past this term was ignored, set to
0., and its inclusion makes a change in X_xrcf, of order +8.1 mm.)
For the current value of F_xrcf = 10258.3 (*3) and using a value of
Node_Hcenter = 470 mm (from Leon's mirror prescription) the
calculation above gives X_xrcf = 8807.2 mm, ideally.  

Based on the XRCF drawings, the nominal effective Rowland Diameter at
XRCF is 8772.8 mm (*4).  The gratings as installed were shimmed
by 0.392" (9.96 mm) towards the HRMA, so the as-built value is 8782.8 mm.

Thus the XRCF grating-focus distance is 24.4 mm (0.96 inches) smaller
(hence the gratings are further from the HRMA) than the ideal value;
this difference exists for reasons of mechanism clearance.  This will
change somewhat the regions illuminated on the gratings as well as
increase slightly the grating vignetting.  These effects must be
measured and included in modeling, but are small enough (< 5%) that
the desired flight calibration accuracy is not in peril.  [A more
detailed calculation gives an additional vignetting of LETG
shell 1 of 3.6% due to this 24.4 mm offset.]


Rowland Diameter Value for XRCF Simulator
- - - - - - - - - - - - - - - - - - - - -
In order to accurately model the gratings at XRCF using csim/MARX it
is necessary to enter the right value into the csim parameter
RowlandDiameter (mm).  In csim RowlandDiameter is the on-axis distance
between the grating-on-axis-vertex and the infinite-source focus
location.  This is given by:

  X_csim = X_xrcf - (F_xrcf - F_flight)

Adopting X_xrcf = 8782.8 mm gives:

  X_csim = 8782.8 - (10258.3 - 10065.5) = 8590.0 mm


Notes
-----
(*1) TRW drawing 301331, sheet 3 gives F_flight = 428.116-31.836 inches
     or 10065.5 mm

(*2) TRW drawing 301331, sheet 3 and 1 yield:
     X_flight = (427.745-56.83+(2.500-1.670) -31.836) = 339.909 inches 
     or 8633.7 mm (as expected).

     TRW drawing 301331 sheet 3 independantly shows on-axis Rowland location as
     X_A = 372.116 which gives X_flight = 372.116-31.836 = 340.282 inches
     This is in error by +0.371 inches which is the distance between the
     HRMA CAP midplane and the HRMA nodal point.  This X_A should be 371.745.

(*3) TRW drawing 301331, sheet 5 indicates XRCF nodal point to focal plane
     is 403.871 inches or 10258.3 mm.

(*4) TRW drawing 301331, sheet 5 yields:
     X_xrcf = 403.5-58.943+(2.500-1.670)  = 345.387 inches 
     or 8772.8 mm

File: ~dd/xrcf/xrcf_X_csim.txt